On the lexicological border
I grew up in the Midwest, where - as Dan rightly says - a "soda" consists of ice cream and a carbonated beverage in a tall, wide-mouthed glass, and is served with a straw and a spoon. The carbonated beverage itself was "pop". Then I moved to eastern Pennsylvania, where "Pop" is your grandfather and the beverage by itself is "soda".
Now I live in the geographic center of PA, which because of the university here has gathered folks from all over... and here "soda" and "pop" are both heard, marking (along with accent) the region of origin.